FIG. 1 of the drawings shows the components of a solder joint 10 formed using FC technology. As will be seen, the solder joint 10 includes a solder bump 12 which is electrically connected to a metal or bond pad 14 of a semiconductor die 16. The solder joint 10 also includes an under bump metallurgy (UBM) layer 18 which serves as a wetting layer for the solder bump 12, and as a diffusion barrier to prevent the ingress of metals/solder into the semiconductor die 16, during various reflow operations including wafer reflow chip join, ball attach and mounting on motherboard
Wafer reflow involves reflowing the solder bump 12 on the die to remove oxides formed on the solder bump 12 so that the bump can be attached to a substrate solder during substrate reflow. Typically, the solder bump 12 has a high-lead content, e.g., the solder bump 12 may be 90% lead (Pb) and 10% tin (Sn), and reflows at a temperature of around 330° C. During wafer reflow some of the Sn present in the die bump and reacts with the UBM layer 18.
During substrate reflow, the solder bump 12 is brought into contact with eutectic solder 20 formed on a substrate 22. Eutectic solder typically comprises about 63% Sn and 37% Pb and has a melting point of 183° C. During chip join to substrate the eutectic solder is heated to its melting point and beyond, e.g. to 220° C. During chip join reflow some of the eutectic solder 20 flows or wicks around the solder bump 12 and reacts with the UBM layer 18. More particularly, the tin from the eutectic solder 20 reacts with nickel from the UBM layer 18, thereby consuming the nickel. This degrades the UBM layer 18, a problem that is exacerbated when subsequent motherboard reflow steps are performed at temperatures between 220° C. and 270° C. At this temperature, the eutectic solder 20 wicks around the solder bump 12, in the manner described above, and reacts with the UBM layer 18 thereby consuming more of the tin in the UBM layer 18. Because the UBM layer 18 has been degraded by the consumption of the tin content therein, as described above, the UBM layer 18 eventually cracks, leading to a failure of the solder joint 10.